Semiconductor devices, such as transistors, are generally formed in monolithic pieces of a semiconducting material, such as silicon. Up to several millions of these discrete devices are then electrically connected one to the other by use of layers of metal thin films. One of the most common metals used for these electrical interconnects is aluminum. Unfortunately, aluminum creates certain problems when in contact with the semiconductor material. For example, when aluminum is in direct contact with silicon, the most commonly used semiconductor material, the aluminum tends to continually diffuse through the silicon during subsequent heating processes. Aluminum diffusion can eventually short out the device formed in the silicon.
To prevent problems such as that described above, so-called barrier layers of metal-based thin films are placed between the aluminum layer and the semiconductor substrate. The barrier layers inhibit the diffusion of aluminum into the semiconductor substrate. This decreases the occurrence of device failure due to aluminum penetration.
One of the materials used as a barrier layer is TiN. TiN is a favored barrier material because of its relatively high density. This characteristic of TiN helps reduce aluminum diffusion because the aluminum atoms cannot migrate easily through the dense TiN thin film.
Because it tends to produce a relatively conformal coating, chemical vapor deposition (CVD) is a highly favored method of depositing TiN. However, CVD TiN may contain a greater amount of carbon than does TiN deposited by certain other methods. The relatively higher carbon content tends to reduce the density of the TiN thin film produced by CVD. The reduction in film density commensurately reduces the effectiveness of the TiN thin film as a barrier layer.
To reduce the amount of carbon in a TiN thin film deposited by CVD, scavenger gases are intermittently introduced into the deposition chamber under conditions which favor a reaction between the carbon in the TiN thin film and the scavenger gases. This removes the carbon from the TiN thin film, and increases the film density.
When a substrate has a relatively planar topology, the scavenger gases can contact all regions of the TiN thin film quite readily. However, when a substrate has relatively deep, narrow channels, such as vias, the scavenger gases do not as readily migrate to the portions of the TiN thin film which are at the bottom of such channels. Unfortunately, a dense TiN thin film is especially desired at the bottom of vias, because the metal interconnect layers contact the semiconducting material at these points.
What is needed, therefore, is a method of improving the quality of a TiN CVD thin film, by removing the carbon inherently deposited in the film, and thus increasing the density of the TiN thin film. What is further needed is such a method which is effectual for TiN thin films which are deposited on substrates having relatively deep, narrow channels.